Peroxisome proliferators are an important class of tumor promoters that bind to members of the nuclear receptor superfamily named PPARs (Peroxisome proliferator-activated receptors). Several subtypes of PPAR have been discovered (alpha, beta, gamma), although the predominant liver subtype (PPARalpha) is our primary interest due to the critical role it plays in carcinogenesis. Little is known about the biochemical properties or post-translational mechanisms that regulate this important receptor. Thus, the central hypothesis to be tested is that PPAR( transcriptional activity is regulated by site-specific phosphorylation and entry into a dynamic chaperone complex containing hsp90 and/or XAP2 The central hypothesis and proposed specific aims are firmly supported by preliminary findings from our laboratories. Four specific aims are proposed: 1). Determine the sites of phosphorylation of PPAR( and examine their role in modulating receptor activity; 2) Examine the role of the co-chaperone protein XAP2 in the regulation of PPAR function; 3). Determine the mechanisms that regulate XAP2 expression; and 4). Examine the role of PPAR chaperones (hsp90, XAP2, hsp70, p23) on transcriptional regulatory complexes. The phosphoprotein PPARalpha is found in cytosolic extracts as a complex with hsp90 and XAP2; both proteins repress PPARalpha activity. This would suggest that regulation of PPAR( activity is quite complex. A series of in vitro and in vivo studies are proposed utilizing a wide variety of techniques including transgenic and knockout mice, immortalized cell lines, transient and stable transfections and protein biochemistry. Taken together the proposed studies will greatly enhance our knowledge of PPARalpha's mechanism of action and hence our understanding of an important class of carcinogen.